Preheat oven to 350°F (175°C). Grease and line a 9×5-inch loaf pan with parchment paper.
Prepare Streusel: In a small bowl, mix flour, brown sugar, and cinnamon. Add cold cubed butter and use a fork, pastry cutter, or your fingers to mix until the mixture resembles coarse crumbs. Set aside.
4 Tablespoons all-purpose flour, ⅓ cup brown sugar, ½ teaspoon cinnamon, 3 Tablespoons cold unsalted butter
Mash Bananas: In a medium bowl, mash the ripe bananas until smooth.
1½ cups mashed ripe bananas
Mix Wet Ingredients: In a large bowl, whisk together the melted butter and sugar. Add eggs and vanilla extract, whisking until smooth.
½ cup unsalted butter, 2 large eggs, 2 teaspoons vanilla extract, granulated sugar
Combine: Stir in sour cream, mashed bananas, salt, and baking soda.
1 teaspoon sea salt, ½ cup sour cream, 1 teaspoon baking soda
Add Dry Ingredients: Gently fold in flour until just combined—do not overmix.
1¾ cups all-purpose flour
Add Blueberries and Nuts: Fold in flour-coated blueberries and optional walnuts.
1 cup fresh or frozen blueberries, Optional: ½ cup chopped walnuts
Bake: Pour batter into the prepared loaf pan. Sprinkle the streusel topping evenly over the batter. Add a few extra blueberries on top if desired.
Bake for 55–60 minutes, or until a toothpick inserted into the center comes out clean. Check at 55 minutes. If the top is browning too quickly, tent loosely with foil.
Cool: Let the bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.